1. Field of the Invention
The present invention relates to an image forming method and apparatus, such as a copier, a facsimile machine, and a printer, and more particularly, to a charging device for charging an image carrying member employed in the image forming method and apparatus.
2. Discussion of the Background Art
As a charging device for charging an image carrying member, a contact-type charging device is known which brings a charging member thereof, such as a charging roller and a charging brush roller, being supplied with a voltage (hereinafter referred to as the charge bias) by contact with the image carrying member.
In such contact-type charging device, if the charging member is supplied with the charge bias only including a direct voltage, uneven charging is more likely to occur than when the charging member is supplied with a voltage including a direct voltage with an alternating voltage superimposed. Consequently, there is a known charging device which uses, as the charge bias, the voltage including a direct voltage superimposed with an alternating voltage.
Even with the voltage including a direct voltage superimposed with an alternating voltage, however, if the alternating voltage has a relatively low frequency, uneven charging may occur in accordance with a number of valleys of the alternating voltage. In the contact-type charging device, therefore, the charging member is supplied with a direct voltage superimposed with an alternating voltage of a relatively high frequency, which is generally approximately 300 Hz (Hertz) to approximately 1000 Hz, so as to attain uniform charging.
Further, a photoconductor used as the image carrying member tends to be partially damaged by electrostatic discharge over time due to electrostatic stress on the photoconductor, if a ratio between a thickness of a surface layer of the image carrying member and absolute value of the surface potential of the photoconductor exceeds 30 V/μm (volts per micrometer). To increase the life of the photoconductor, therefore, preferably the photoconductor is used with the surface potential thereof set to a not excessively large value.
Further, a so-called cleaner-less image forming apparatus is known which does not include such devices as a cleaning blade for cleaning post-transfer residual toner remaining on the image carrying member after a transfer operation, and which collects the post-transfer residual toner into a development device. In the cleaner-less image forming apparatus, due to the potential difference between the surface potential of a non-image area of the image carrying member and a bias voltage applied to the development device, the post-transfer residual toner remaining on the non-image area of the image carrying member after the transfer operation is returned to the development device in a development step of the next and subsequent processes, i.e., when the image carrying member is again charged, exposed, and developed.
According to the method, the post-transfer residual toner is returned to the development device and reused in the development of an electrostatic latent image in the next and subsequent processes. It is therefore possible to eliminate waste toner and to reduce maintenance labor. Further, the absence of a waste toner container is advantageous in that it reduces the size of the image forming apparatus. Furthermore, it is possible to reduce the load on the image carrying member caused by the cleaning blade, and thus to improve the durability of the image carrying member.
In the use of the contact-type charging device, however, the residual toner on the image carrying member tends to adhere to the charging member when passing through a charging nip portion formed between the image carrying member and the charging member. If the adhesion repeatedly occurs and more than an allowable amount of toner adheres to the charging member, defective charging is caused. In particular, in the above-described cleaner-less image forming apparatus, the image carrying member passing through the charging nip portion carries thereon a relatively large amount of the residual toner. Thus, the adhesion of the toner to the charging member is remarkable.
In view of the above, an attempt has been made to clean the charging member by changing the charge bias in a non-image forming area immediately after the completion of a print job or between sheets during successive feeding of sheets in a continuous printing operation, for example, to thereby form an electrical field for causing the post-transfer residual toner adhering to the charging member to move to the image carrying member, and thus to transfer the post-transfer residual toner again to the image carrying member.
In one example of the above-described method, the image forming apparatus employs a negatively charged image carrying member and toner of negative polarity. In an image forming area, a charge bias generated by superimposing an alternating voltage on a direct voltage of the negative polarity is applied to the charging member to charge the image carrying member uniformly, i.e., to a constant value. Meanwhile, in a non-image forming area, the direct voltage of the charge bias applied to the charging member is set to be larger toward the negative polarity than the surface potential of the image carrying member. Due to the difference between the charge bias and the surface potential of the image carrying member, an electrical field is formed which causes the toner of the negative polarity adhering to a surface of the charging member to move to the image carrying member. Therefore, the toner adhering to the charging member is moved to the image carrying member. In the charge bias applied in the non-image forming area, the direct voltage is larger toward the negative polarity than the surface potential of the image carrying member, and the alternating voltage may be set to zero volts.
The image carrying member before entering the charging nip portion carries thereon toner of the reverse polarity (i.e., the positive polarity in the present example) in which the polarity has been reversed by the transfer process and weakly charged toner as well as the toner of the negative polarity. Such types of toner also adhere to the charging member. On the charging member, such types of toner are gradually charged to the negative polarity due to the electrical discharge or injection from the charging member, which has been supplied with the charge bias of the negative polarity in the image forming area. The toner on the charging member charged to the negative polarity is moved to the image carrying member by the electrical field, which is formed by the alternating voltage applied to the charging member and which causes the toner adhering to the charging member to move to the image carrying member. However, the toner, once adhered to the charging member, is difficult to move. Further, in the image forming area, the magnitude of the electric field is approximately half the peak-to-peak value Vp-p of the alternating voltage, and is not sufficiently large. As described above, therefore, a relatively large direct voltage of the negative polarity is applied to the charging member in the non-image forming area so as to increase the electric field and thus move the toner to the image carrying member.
Further, to return the post-transfer residual toner on the image carrying member to the development device in the cleaner-less image forming apparatus, it is necessary that the post-transfer residual toner carried on the image carrying member has a normal charge polarity when the image carrying member has passed through the charging nip portion and arrived at a position facing the development device, and that the toner has been charged to a degree enabling the development device to develop an electrostatic latent image on the photoconductor. However, the reversely charged toner and the weakly charged toner cannot be returned from the image carrying member to the development device, and thus cause a defective image.
To prevent such a defect, it is effective for the cleaner-less system to charge the reversely charged toner and the weakly charged toner to the normal charge polarity on the charging member, as described above, and then to cause the toner to move onto the image carrying member and reach the position facing the development device.
However, in the image forming apparatus as described above, which changes the direct voltage to form the electric field for causing the post-transfer residual toner adhering to the charging member to move to the image carrying member, the electric field is changed by the amount of change of the direct voltage. It is difficult to apply a substantially large direct voltage without adversely affecting the longevity of the image carrying member. Meanwhile, if the above-described electric field is formed with the direct voltage changed within a practical range, it is difficult to move the toner adhering to the charging member effectively to the image carrying member.